We uninstalled Win-Defender, and installed Avast Business Antivirus, version 18.5.2535
It picked up the test-virus e-mail immediately and moved it to the Virus Chest.
So far I am impressed with the simplified installation / setup and easy of use.
We will run it on a 30 day trial, the cost is also Very reasonable.
it's depends up on your budget...
you can get best Mcafee, Symantec, Kaspersky etc. server versions
as well after configuration it needs to configure the Exclusion on the antivirus as well
ex: https://docs.microsoft.com/en-us/exchange/antispam-and-antimalware/windows-antivirus-software
if you wnat to install antispam as well ..i.e which comes with Mcafee ... you can install Antispam and antivirus both on 1 machine ..
all the best